This invention relates to a bobbinless solenoid coil and to a method for manufacturing such a coil, particularly a large superconducting solenoid for use in nuclear fusion devices and having a diameter on the order of 3 meters.
In very large and specialized coils of this type it is desirable if not essential to avoid the use of a permanent inner bobbin in order to minimize the material thickness of the solenoid, and the exterior of the coil must be supported by a rigid cylindrical casing in order to withstand the extremely high, outwardly directed electromagnetic forces generated during the use of the solenoid. The current carried by the coil windings is on the order of 3,000-5,000 amperes. The dimensional tolerances of the coils are very precise and critical, and both a tight and uniform contact must be established between the outer surface of the coil and the surrounding support cylinder to implement the necessary structural support and to enable effective cooling in view of the very large quantities of heat generated by the current flowing through the coil. All of these constraints present very specialized and difficult manufacturing problems as contrasted with small and conventional solenoid coils which can be wound on permanent inner bobbins in a fully automated manner and which can easily and inherently withstand the relatively low magnitude electromagnetic forces developed during operation without resort to any external support cylinder.
One prior art approach to the fabrication of these large and specialized solenoid coils is illustrated in FIG. 1 and described in greater detail in volume 45 of the Journal de Physique of January 1984 on pages 333-336. Essentially, a coil 1 is wound around an inner mandrel 2, an outer support cylinder 3 is heated to enlarge its inner diameter, is inserted over the coil 1, and is thereafter allowed to cool to effect a shrink fit. The mandrel 2 is then disassembled and removed. With such a fabrication technique the degree of shrinkage of the cooling support cylinder is somewhat unpredictable and difficult to control, however, which leads to inaccuracies and excessive rejects. Too little shrinkage results in poor and non-uniform contact between the cylinder and the coil; too much shrinkage results in the deformation of the coil windings.
A further prior art technique for the fabrication of these superconducting solenoids is generally illustrated in FIG. 2 and described in greater detail on pages 337-340 of the aforementioned Journal de Physique volume. In this technique the insulated conductor is first wound about a temporary or dummy mandrel 4. The latter is then disposed inside the support cylinder 3 and the conductor is applied against the inner surface of the cylinder under compressive stress to thereby form the coil 1. With this procedure it is difficult to accurately control the axial or compressive stress applied to the conductor during the "transfer" winding, however, which leads to non-uniform contact adhesion between the coil 1 and the cylinder 3. Too much stress leads to the buckling of the conductor; too little stress results in contact gaps and discontinuities.